The present invention relates to motor control systems and, more specifically, to motor control systems for variable speed and variable torque induction motor control.
In mobile applications, there is quite frequently the need for variable speed and variable torque motors such as, for example, for winches and hoists. DC motors have been employed in such applications due to the general availability of battery power in mobile vehicles. DC power is less than ideal due to the fact that it is customarily at low voltage, resulting in high copper losses in the supply line as well as the fact that most DC systems generate substantial electrical noise which is undesirable in the presence of sensitive electronic equipment.
In certain applications, and in particular, in airborne applications, high power AC sources of power are generally available at a constant frequency of, for example, 400 Hz. Although it is tempting to consider the use of AC motors in such mobile applications, the ability to achieve variable speed with a constant frequency source has been limited.
In induction motors of the squirrel-cage type, it is possible to control motor speed by varying the voltage of the applied 400 Hz.
As is well known, squirrel-cage induction motors depend on the slip between the rotating magnetic field and the rotating rotor. With a fixed frequency AC source, the only way of varying motor speed is to permit the slip to vary widely. It is also well known that maximum torque, resulting in maximum motor efficiency, occurs at a slip that is a relatively small fraction of the synchronous speed of the motor. As the slip is permitted to increase significantly in order to provide variable motor speed, the motor efficiency becomes poorer and poorer.